Metabolism of 2-carboxyarabinitol l-phosphate (CA l-P), an endogenous inhibitor of ribulose. bisphosphate carboxylase/ oxygenase, occurs in the light. A soluble protein fraction which metabolized CA 1-P in the presence of NADPH was isolated from tobacco chloroplasts. A similar fraction from spinach exhibited much lower activity. The activity in tobacco extracts was stable overnight at 4°C but its maintenance during storage required dithiothreitol. The tobacco protein responsible for CA 1-P metabolism was partially purified by ion-exchange FPLC of stromal extracts. The requirements for NADPH and dithiothreitol for activity of this protein suggest a mechanism for the light-dependent control of CA 1-P levels in plants.

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The catalytic degradation of 2-carboxyarabinitol 1-phosphate (CA 1-P), a naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), was investigated by chromatographic and spectroscopic analyses of the reaction products. Carboxy-labeled [14C]CA 1-P was incubated with a partially purified tobacco (Nicotiana rustica) chloroplast protein that has been shown previously to catalyze metabolism of CA 1-

The regulation of 2-carboxyarabinitol 1-phosphatase (CA 1-Pase) by phosphorylated effectors was studied with enzyme purified from tobacco (Nicotiana tabacum) leaves. CA 1-Pase activity was most stimulated by fructose 1,6-bisphosphate, exhibiting an Ao.5 value of 1.9 millimolar and a 10-fold enhancement of catalysis. With nbulose-1,5-bisphosphate, the AO.5 was 0.6 millimolar, and maximal stimulation of activity was 5.3-fold. Among the monophosphates, 3-phosphoglycerate and phosphoglycolate were more potent positive effectors than glyceraldehyde 3-phosphate, glucose 1-phosphate, glucose 6-phosphate, and dihydroxyacetone phosphate. Stimulation of CA 1-Pase by ribulose-1,5-bisphosphate and fructose 1,6-bisphosphate increased Vmax but did not appreciably alter Km (2-carboxyarabinitol 1-phosphate) values. Inorganic phosphate appeared to inhibit CA 1-Pase noncompetitively with respect to 2-carboxyarabinitol 1-phosphate, exhibiting a K, of 0.3 millimolar. The results suggest that these positive and negative effectors bind to a regulatory site on CA 1-Pase and may have a physiological role in the light regulation of this enzyme. Related experiments with CA 1-Pase inactivated by dialysis in the absence of dithiothreitol show that partial reactivation can be achieved in the presence of a range of reducing reagents, including dithiothreitol, cysteine, and reduced glutathione. This could imply an ancillary involvement of sulfhydryl reduction during light activation of CA 1-Pase in vivo. The enzyme was thermally stable up to 350C, in contrast to ribulose-1,5-bisphosphate carboxylase/oxygenase activase which lost activity above 300C. The activation energy for CA 1-Pase was calculated to be 56.14 kilojoules per mole.

During a shipboard expedition to Andros Island (Bahamas), photosynthetic measurements for the macroalgae Cladophoropsis membranacea (Chlorophyta), Dilophus guineensis, Turbinaria turbinata, and Lobophora variegata (Phaeophyta), and Laurencia papillosa (Rhodophyta), taken directly from their marine habitat, showed that only Cladophoropsis was saturated at seawater inorganic carbon levels (2.5 mM). The photosynthetic k0.5 values for inorganic carbon ranged from 1.1 to 3.2 mM. Decreasing the pH at 2.5 mM inorganic carbon, and thus enhancing the CO2 by 30-fold, only slightly increased photosynthesis, suggesting that bicarbonate was the major assimilated form of inorganic carbon. At 2.5 mM inorganic carbon, only Lobophora exhibited a Warburg effect on photosynthesis (49%), but at 0.5 mM, Turbinaria and Laurencia were also inhibited by O2. Ribulosebisphosphate carboxylase–oxygenase appeared to be the predominant carboxylation enzyme, but in Dilophus and Laurencia extracts, its activity was rivaled by phosphoenolpyruvate carboxylase and carboxykinase. Malate pools were detected in Turbinaria and Laurencia, and in the latter they were greater at night than during the day. However, this diel fluctuation was too small to implicate crassulacean acid metabolism. The data indicate that the bicarbonate concentration in seawater is insufficient to overcome O2 inhibition effects on photosynthesis, unless the macroalga has some form of CO2 concentrating system, based on bicarbonate uptake or C4 acid metabolism. In addition, the inorganic carbon in seawater may be a nutrient limiting the photosynthesis and productivity of certain macroalgae.